In the field of wireless communications, the hybrid automatic repeat request (HARQ) technique is typically used in a wireless communication system to resend data packets that cannot be decoded successfully to improve the system throughput. A HARQ process is activated when a new data packet is transmitted. In a typical implementation of the HARQ process, each data packet to be transmitted by a transmitter is attached with a cyclic redundancy check (CRC) code for error detection. At a receiver, the contents of each received packet are validated using CRC. If the received packet fails the CRC validation, the receiver sends a non-acknowledgment (NACK) signal back to the transmitter to request for a retransmission. The packets are retransmitted until either they are decoded successfully or until the maximum number of retransmissions (e.g. 4 retransmissions) is reached. Otherwise, if the received packet is successfully validated using the CRC validation, an acknowledgement (ACK) signal is sent back to the transmitter to acknowledge correct decoding of the data packets. At the receiver, the received retransmitted packets and the received originally transmitted packets are combined to improve the system throughput.
Using current technologies, there are two typical protocols used to implement the HARQ process, a Chase combining protocol and an Incremental Redundancy (IR) protocol. Using the Chase combining protocol, a data packet (ie. a Chase packet) substantially identical to an originally transmitted data packet which contains system information and some parity information, is retransmitted by the transmitter when it receives a retransmission request. On the other hand, using the IR protocol, the transmitter transmits a data packet comprising system information and some parity information in the initial transmission. When the initial data packet fails the CRC validation and retransmission is requested, more parity information is typically transmitted in a retransmitted packet (ie. an IR packet) to provide more redundancy to assist in decoding of the system information. The parity information in the retransmitted packet is different from the parity information contained in the originally transmitted packet. Based on the above, in contrast to the Chase combining protocol, the retransmitted IR packet is not a repetition of the originally transmitted packet.
With regards to the capacity of the wireless communication system, the capacity is typically enhanced by using simultaneous transmissions of multiple data streams in a multiple input multiple output (MIMO) communication system that employs multiple transmit (NT) antennas and multiple receive (NR) antennas. The MIMO system is typically configured either to improve performance through transmit diversity or to increase system capacity by using spatial multiplexing (SM).
Transmit diversity is typically achieved by using space-time block coding (STBC) which provides space and time diversity. The STBC is described in “Space-Time Block Codes from Orthogonal Designs”, IEEE Transactions on information theory, Vol. 45, pp. 1456-1467, July 1999 (by Tarokh, V., Jafarkhani, H., Calderbank, A. R.) and in WO 99/15871. Increasing system capacity by using SM is described in “V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel” in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, Sep. 29 to Oct. 2, 1998 (by P W Wolniansky et al.).
In addition, MIMO techniques are typically used in conjunction with orthogonal frequency division multiplexing (OFDM) to achieve more efficient spectral utilization by transferring frequency selective fading channels into a set of parallel frequency-flat fading and orthogonal subchannels overlapping each other.
In the MIMO system, when multiple data streams are transmitted in parallel over multiple antennas using spatial multiplexing, data streams transmitted over different antennas typically have different error performances since the streams experience different link conditions. It has been recognised that it is typically unlikely that the data streams experience detection errors simultaneously, especially when a large number of antennas are employed. This antenna diversity may be used to further improve the system throughput of the MIMO system by employing an antenna-dependent HARQ technique. The antenna-dependent HARQ technique comprises using independent HARQ processes for independent data streams transmitted over different antennas. At the receiver, each received data stream may go through an independent CRC validation. Multiple ACK/NACK indications may then be sent by the receiver back to the transmitter. The transmitter may retransmit data streams based on the ACK/NACK indications. Hence, in this way, the system throughput of the MIMO system may be further increased since only the transmit antennas receiving NACK feedback signals retransmit packets while the transmit antennas receiving ACK feedback signals transmit new packets.
However, since the link conditions may not vary at a fast rate over two consecutive transmission intervals, the transmission quality of the data stream which fails the CRC validation in the previous transmission may not be improved. Hence, in the antenna dependent HARQ retransmission scheme discussed above, one critical issue is that a higher number of retransmissions may be required to achieve successful decoding and CRC validation of the system information.
To improve the transmission quality of the retransmitted data streams when the above antenna dependent HARQ technique is used, the retransmitted data streams and the newly transmitted data streams may be encoded using STBC to take advantage of the transmit diversity typically offered by STBC. The STBC encoding may improve the transmission quality of both the data streams to be retransmitted and the new data streams.
In view of the above, preferred embodiments of the present invention described herein provide a method for retransmission of data in a MIMO system to address at least the issue.